I understand the suggestion
https:// www.truenas .com/community/threads/grub-minimal-bash-need-help-to-fix-broken-boot.108977/
is valid and “easy” - however this is the third time this has happened to me.
It is frustrating that “something happens to my server”, I have no logs/warning about it, and, when it (auto-)restarts it is a bit trashed and I need to recover it all over again!
I have changed to new drives - and EVEN set those drives to RAID1 (… or whichever method the TrueNAS installer allows to use TWO drives for booting).